1. Field of the Invention
The present invention relates to explosive compositions comprising a sensitized blend of a water-in-oil emulsion and solid particulate inorganic nitrate, preferably ammonium nitrate (AN), in the form of prills or granules which may be coated with fuel oil (e.g., ANFO), and more particularly to such compositions in the form of storage-stable packaged products and bulk products adapted to be pumped into boreholes. The invention also relates to a low-viscosity emulsion particularly adapted to be blended with fuel-free or -deficient solid inorganic nitrate to form such a blend.
2. Description of the Prior Art
Explosives which comprise a blend of a water-in-oil emulsion and solid particulate AN (e.g., ANFO) have captured the interest of blasters in recent years owing to the fact that they are able to offer the advantages of high bulk density, blasting energy, and water resistance characteristic of emulsion explosives, while at the same time resulting in cost reductions owing to the lower cost of the AN. Among the problems that may be encountered in connection with the use of these blends, however, are those of blend pumpability and blend stability, more particularly of the stability of the blend's explosive properties. Some blends are not pumpable, or only difficulty pumpable. Some must be pumped immediately after they have been formed because they do not retain their pumpability even for a day or two. While there is no question but that the blend must have a sufficiently long shelf life as to be detonable after it has been emplaced in a borehole, this matter has not been dealt with to any significant degree in most of the prior art sources on emulsion/AN blends. Nevertheless, it is a fact that not all packaged blends are detonable by the time they are to be used, even if the packages have been stored for only a short time.
Emulsion/AN blends are described in U.S. Pat. Nos. 3,161,551 (Egly et al.); 4,111,727 (Clay); 4,181,546 (Clay); and 4,357,184 (Binet et al.), and British Pat. No. 1,306,546 (Butterworth). Egly et al. describe an emulsion/AN blend wherein the emulsion, said to be in a sensitized form, is employed as a sensitizer for the solid ammonium nitrate. Regarding the delivery of the blend into a borehole, the patentees describe forming the blend in the borehole itself, i.e., by dropping the AN into the hole and pouring the sensitized emulsion over it.
Clay, whose 10/90 to 40/60 emulsion/AN blends in U.S. Pat. No. 4,111,727 are sensitized only by the air entrapped in the AN, states that the emulsion and AN particles are combined by very simple procedures, preferably just prior to insertion into the borehole. Clay also states that sorbitan monooleate, sorbitan monostearate, and sorbitan monopalmitate are quite suitable emulsifiers for making his emulsion, and that the emulsifiers preferably are blended into the oil before the aqueous component is added. Clay's AN may be oxygen-balanced ANFO (to be blended with an oxygen-balanced emulsion), or fuel-deficient or fuel-free solid AN (to be blended with an emulsion that contains most or all of the oil required to oxygen-balance the blend).
In U.S. Pat. No. 4,181,546, Clay describes 40/60 to 60/40 emulsion/AN blends having completely filled interstices in and between the AN particles. This product is said to contain too high a proportion of dry ingredient to be pumpable in conventional slurry pumps, but is said to be deliverable to a borehole by an auger in the same manner as dry ANFO. This patent advises minimizing the amount of emulsifier, and using high shear mixing, to insure a stable emulsion. Clay describes sorbitan fatty acid esters as being particularly suitable emulsifiers, and "Glycomul 0" (sorbitan monooleate) as superior to most for his invention.
Butterworth describes loading his blend into an 8.3-cm-diameter polyethylene tube, priming the charge with nitroglycerin, and detonating the charge one hour after mixing. Thus, Egly et al., Clay, and Butterworth do not address themselves to such matters as blend stability, i.e., the condition of the blend after it has been allowed to stand for several days or weeks before or after packaging, or before delivery in bulk form to a borehole.
The emulsion portion of Binet et al.'s explosive composition is termed a "microemulsion", and it contains an amphiphatic synthetic polymer emulsifier, along with a conventional water-in-oil emulsifier. Optionally, a phosphatide emulsion stabilizer is included. Binet et al.'s microemulsion per se, described as a "liqui-liquid foam" of very small cell size ranging from less than 1 micron to about 15 microns, is said to display exceptional long-term storage stability and to be tolerant to doping with further fuel and energy-enhancing ingredients. The patentees discuss a destabilizing seeding crystal effect in prior art emulsion explosives resulting from the presence of solid oxidizer salts in the basic emulsion. According to Binet et al., their findings show that their microemulsion, when doped with 24 percent ground AN, was much more stable to this seeding crystal effect than a prior art emulsion, and remained cap-sensitive for three cycles, each consisting of 3 days of storage at 50.degree. C. followed by 2-3 days at -17.degree. C.
Binet et al.'s consideration of storage stability is directed for the most part at the explosive emulsion itself. The patentees mention that all known prior art water-in-soil emulsions suffer from lack of stability owing to the seeding effect. Binet et al. also imply that the seeding effect is a problem in AN-doped emulsions, although they do not explain how this can be so in microemulsions containing relatively large AN particles. Moreover, Binet et al. require an expensive polymeric emulsifier, and an optional emulsion stabilizer, to achieve improved stability in their microemulsion.
AN/emulsion blends having good storage stability, and a method of making such blends which does not require the use of expensive additives, of perhaps limited utility, are greatly needed to expand the spectrum of AN/emulsion products that can be made available to the public. In particular, blends are needed which are pumpable into a borehole even a few days after having been formed, as well as detonable after having been delivered into a borehole in packaged form after a period of about three months of more from the time the blends were made.